1. Field of the Invention
The present invention relates to a plasma address liquid crystal display device provided with a plasma cell and a liquid crystal cell.
2. Description of the Related Art
FIG. 1 is a front view showing a plasma address liquid crystal display device and FIG. 2 is its cross-sectional view. As shown in FIGS. 1 and 2, the plasma address liquid crystal display device comprises a transparent first substrate 1 and a transparent second substrate 2 each of which is made of, for example, a glass substrate and which are opposed to each other. Between the first and second substrates 1 and 2, there are provided a liquid crystal cell 3 and a plasma cell 4 in a superimposed fashion.
The plasma cell 4 is formed in such a manner that a transparent third substrate 5 made of, for example, a thin glass plate is sealed to the inner surface of the second substrate 2 with a predetermined interval therebetween in an air-tight by a frit 6 to define a narrow flat air-tight space between the second substrate 2 and the third substrate 5.
In the space there are formed in parallel a plurality of partition walls 7 each of which has a stripe-like shape and is extended in one direction, for example, horizontal direction. In the space between the adjacent partition walls 7 and on the inner surface of the second substrate 2, formed in parallel are a first electrode 8 and a second electrode 9 each of which has a stripe-like shape and which form a pair of discharge electrodes. A discharge gas is sealed in the space of the plasma cell 4.
FIG. 3 is a cross-sectional view showing the main portion of the liquid crystal cell 3 in an enlarged scale. As shown in FIG. 3, in the liquid crystal cell 3, there are sequentially arranged color filters R, G and B of red, green and blue in parallel each of which has a stripe-like shape and extended in a direction to cross the extending direction of the first and second electrodes 8 and 9 in the plasma cell 4 described in connection with FIG. 2, for example, in the vertical direction. A black stripe BL is formed between adjacent ones of the respective color filters R, G and B. A stripe-like shape transparent electrode 10 is formed along each of the color filters R, G and B in the extending direction of the black stripe BL.
The first substrate 1 having formed thereon the respective color filters R, G and B and the transparent electrode 10 is bonded to the third substrate 5 at its outer peripheral portion with a predetermined interval therebetween by an adhesive agent 11 in a liquid-tight, and further bonded to the second substrate 2 outside the first-mentioned bonding portion by an adhesive agent 11. A liquid crystal is charged into the flat liquid-tight space between the first substrate 1 and the third substrate 5, whereby the liquid crystal cell 3 is formed.
Every other transparent electrode 10 of the liquid crystal cell 3, for example is extended at its both ends to form a terminal T on both projected sides of the first substrate 1 beyond the second substrate 2 in the vertical direction as shown in FIG. 1. The terminals T are set as column terminal groups 12 to which a drive voltage in the horizontal direction is applied.
One electrode, serving as an anode electrode of the electrodes of the plasma cell 4 is extended at its one end to one side of the second substrate 2 beyond the first substrate 1 in the horizontal direction and is set as row terminal groups 13 to which a drive pulse in the vertical direction is applied.
The respective terminals of the column terminal groups 12 are connected to the corresponding terminals of a column substrate 14, on which a column drive circuit is formed, through flexible cables 15, respectively.
Similarly, the respective terminals of the row terminal groups 13 are connected to the corresponding terminals of a row substrate 16, on which a row drive circuit is formed, through flexible cables 17, respectively.
On the outer surfaces of the first and second substrates 1 and 2 there are respectively provided film-like polarizing plates 18 and 19 whose polarizing axes cross at right angle with each other.
In the plasma cell 4 thus arranged, a plasma discharge is sequentially generated in the plasma spaces defined between the adjacent partition walls 7 to make the third substrate 5 serve as an anode electrode. On the other hand, when a predetermined voltage in accordance with a display signal is sequentially applied to the transparent electrodes 10 in the liquid crystal cell 3, a potential difference is generated at that position across the liquid crystal and hence at that position a potential is written in the liquid crystal. Therefore, if such a state is assumed that no potential difference is applied between both the surfaces of the liquid crystal, for example, and almost all the light from a light source disposed on the rear side of the second substrate 2 can not pass through both the polarizing plates 18 and 19, by the writing of the potential to the liquid crystal mentioned above, the linear polarized light from the polarizing plate 19 is made to pass through the polarizing plate 18, whereby a target optical image can be displayed.
By the way, in the above-mentioned display device, for example, on both outsides of the picture screen in the horizontal direction, there are caused a distortion in the picture screen and an unstable display due to a continuous use in a long period of time and so on.
It was ascertained that the above-mentioned distortion and unstableness of the picture screen were caused by the transparent electrode formed in an ineffective picture screen area outside the effective picture screen area of the display device mentioned above.
In the liquid crystal cell of the display device, in order to prevent any display irregularity from being generated, it is necessary that the thickness of liquid crystal at each display position in the liquid crystal cell should be uniform. In other words, the distance or interval between each of the transparent electrodes 10 and the third substrate 5 has to be set uniform. The interval is set in such a manner that after the respective color filters R, G, B and the transparent electrodes 10 are formed as shown in FIG. 3, particle-like spacers 20 each having a predetermined diameter are dispersed to set the above interval to be a predetermined one. In order to set the interval accurately, it is also necessary that a portion having a height equal to that of the transparent electrode 10 must be formed on the peripheral portion of the liquid crystal cell 3. To this end, in this kind of liquid crystal display device, the transparent electrode 10 is formed not only in the effective picture screen area but also in the ineffective picture screen area around the former at the same time when the light absorbing layer BL disposed between the adjacent color filters R, G and B is formed, and also a dummy light absorbing layer BLD made of the same material and the same thickness as those of the light absorbing layer BL is also formed. On the dummy light absorbing layer BLD, at the same time when the transparent electrode 10 is formed, an ineffective transparent electrode 10D is formed of the same material as that of the transparent electrode 10 and which has the same thickness as that of the transparent electrode 10.
The ineffective transparent electrode 10D is provided so to set the interval only so that any external potential is not applied thereto and it is floated in electrical point of view.
On the contrary, if an electric field is fixedly applied to the liquid crystal in one direction for a long period of time, the composition of liquid crystal is decomposed or the like to thereby cause deterioration of picture quality and life span. As schematically shown by arrows a in FIG. 6, inverse voltages are applied to the transparent electrode 10 so that the liquid crystal driving is inverted.
Since the ineffective transparent electrode 10D is not electrically connected to the outside at all and hence electrically floated, as shown in FIG. 6, between the ineffective transparent electrode 10D and the adjacent transparent electrode 10 which is located on the most outside of the effective picture screen there is generated an electric field b different from the inherent electric field a. Further, the ineffective transparent electrode 10D is apt to be set to a state in which an unstable potential is applied thereto by the electrification in the first substrate 1 or the like. Therefore, it is clarified that an unstable display is carried out near the side edge of the effective picture screen.